1. Field of the Invention
The present invention relates to a display panel driver for driving a liquid crystal display panel or the like, and particularly to a display panel driver having a voltage step-up circuit
2. Description of the Prior Art
A display panel such as a liquid crystal display device is driven by a driver IC (integrated circuit) to achieve display. Such a driver IC is supplied with a power source voltage by a battery, and is controlled by a microcomputer to produce driving signals.
The driver IC includes a voltage step-up circuit for stepping up the power source voltage. The produced stepped-up voltage is fed to a display voltage producing circuit.
The voltage step-up circuit steps up the power source voltage in such a way that voltages stepped-up by different factors appear at different ports for output.
This conventional display panel driver, however, has the following disadvantage. The power source voltage supplied from the battery to the driver IC varies with the type of the battery. Accordingly, within the driver IC, to allow the display voltage producing circuit to receive an appropriate voltage in any situation, the voltage step-up circuit is provided with a plurality of output ports to output a plurality of voltages stepped-up by different factors.
For example, consider a case where the battery voltage Vin is 3.3 V and this voltage needs to be stepped up by a factor of three by the voltage step-up circuit to obtain a display voltage of 8.0 V from the display voltage producing circuit. In this case, if replacing the battery, for example, ends in reducing the power source voltage to 2.0 V, then it is impossible to obtain a voltage higher than 6.0 V simply by stepping up the power source voltage by a factor of three, and thus it is impossible to achieve display on the display panel.
One way to obtain a display voltage of 8.0 V at the maximum even when the battery voltage is 2.0 V is to set the voltage step-up circuit to step up the voltage by a factor of five, feed the stepped-up voltage to the display voltage producing circuit, and adjust it to 8 V by the use of an appropriate adjusting circuit. In this case, however, if the battery voltage Vin is 3.3 V, the voltage step-up circuit steps it up by a factor of five, that is, to 16.5 V. Such excessive stepping up of the voltage leads to unduly high current consumption.